Evaluation of Injectable Anesthetics for Major Surgical Procedures in Guinea Pigs

REPORTS Evaluation of Injectable Anesthetics for Major Surgical Procedures in Guinea Pigs KELVIN C. BUCHANAN, DVM, MPH, 1 ROBERT R. BURGE, MS, 2 AND GAYE R. RUBLE, DVM, MPH 3 Abstract Anesthetizing guinea pigs is difficult with varying outcomes. The primary purpose of the study reported here was to evaluate six injectable anesthetic regimens for use in guinea pigs and assess the depth of anesthesia and, thus, their effectiveness in terms of their use for major surgical procedures. Other variables that were measured and evaluated included time from injection until onset of anesthesia, duration of anesthesia, depth of anesthesia, and vital signs (i. e., respiratory rate, heart rate, and body temperature). Female Dunkin Hartley guinea pigs that were 9 to 12 weeks old were randomly assigned to receive one of the following anesthetic regimens: ketamine/xylazine (KX), ketamine/detomidine (KD), ketamine/medetomidine (KM), 4) tiletaminezolazepam/xylazine (TX), tiletamine-zolazepam/detomidine (TD), or tiletamine-zolazepam/medetomidine (TM). All anesthetics were administered intramuscularly. Anesthesia was assessed by attempting to perform an ovariohysterectomy. Surgery could not be performed on any guinea pigs in the groups given ketamine or TD. There was a high rate of adverse effects in guinea pigs receiving detomidine. Four of six guinea pigs in the TD group died during or after the anesthetic episode. Fourteen of 30 (46.7%) guinea pigs given TX underwent successful surgery, and 23 of 29 (79.3%) given TM underwent successful surgery. A combination of tiletamine-zolazepam and xylazine or medetomidine was effective for inducing anesthesia and providing sufficient analgesia to perform a major surgical procedure on guinea pigs. However, TM was the most reliable regimen. Guinea pigs (Cavia porcellus) are widely used in biomedical research. However, they are one of the most difficult species of rodents to anesthetize (1). Reasons for this are numerous. Routes of administration are limited, and medications can be administered intravenously only in the lateral saphenous, aural, or dorsal penile veins (2). However, these vessels are small in guinea pigs and thus, it can be difficult to administer anesthetics intravenously. Inhalant anesthetics require additional equipment and resources that are not always readily available. Therefore, a safe, effective, and reliable injectable anesthetic that can be administered intramuscularly or intraperitoneally is needed. Another reason that guinea pigs are difficult to anesthetize is their extremely variable response to most anesthetic agents (3 9). Green et al. reported that use of a combination of ketamine hydrochloride (60 mg/kg of body weight) and xylazine hydrochloride (8 mg/kg) resulted in guinea pigs that retained pain and corneal reflexes (9). However, Kellner reported safe and adequate anesthesia when using a similar regimen (ketamine, 60 mg/kg and xylazine, 4 mg/kg) (4). Flecknell has reported that a combination of fentanyl/fluanisone and diazepam produces reliable anesthesia (6), but Whur (7) and Suckling (5) have reported the inability to achieve sufficient anesthesia to allow surgery with that combination. Some injectable anesthetic regimens appear to be effective for inducing anesthesia and providing analgesia (10 14); however, many of these agents are not readily available in the United States (12 14). In most studies, investigators evaluate analgesic or anesthetic effectiveness by monitoring vital signs and loss of reflexes (3,9,11,14 16). However, there may be instances in which many reflexes are abolished, yet analgesia is insufficient to enable investigators to perform a major surgical procedure. In only a few studies have researchers specifically evaluated the usefulness of anesthetics when performing surgical procedures on guinea pigs (12, 13). Ketamine is classified as a dissociative anesthetic that causes a 1 Armed Forces Radiobiology Research Institute, Veterinary Sciences Department, 8901 Wisconsin Avenue, Bethesda, MD 20889-5603, 2 Division of Biometrics and 3 Division of Veterinary Medicine, Walter Reed Army Institute of Research, Washington, D.C. 20307-5100 cataleptoid state accompanied by cardiovascular and respiratory stimulation, maintenance of normal pharyngeal and laryngeal reflexes, and some superficial analgesia. Ketamine used alone induces stage-i and -II anesthesia, but not stage-iii anesthesia, and is considered insufficient for major surgical procedures (17). Analgesia is poor with ketamine alone, even with dosages as high as 250 mg/kg (9). Tiletamine hydrochloride-zolazepam hydrochloride is a potent combination product. Tiletamine is also classified as a dissociative anesthetic, with pharmacologic properties similar to those of ketamine. Tiletamine in guinea pigs reportedly causes only CNS depression without anesthesia (17). Zolazepam is a non-phenothiazine diazepinone that has minor tranquilizing properties. Xylazine, detomidine, and medetomidine are all classified as non-narcotic 2 -adrenoreceptor agonists that cause sedation, provide superficial and visceral analgesia, and potentiate the effects of general anesthetics. These drugs differ in potency, with xylazine being the least potent. Detomidine is approximately 40 times more potent than xylazine (18). Medetomidine is 10 times more potent than detomidine for inhibiting spontaneous locomotor activity in rats. It also has an 2 : 1 -adrenoreceptor selectivity of 1,620, compared with 260 for detomidine (19). The goal of the study reported here was to find an effective anesthetic regimen that abolished specific reflexes, and, more importantly, provided sufficient anesthesia and analgesia to allow investigators to perform a major surgical procedure (i. e., an ovariohysterectomy). Materials and Methods Animals: Ninety-two specific-pathogen free female Dunkin Hartley guinea pigs (Harlan Sprague Dawley, Inc., Indianapolis, IN) that were 9 to 12 weeks old and weighed between 403 and 806 g were used in the study. All procedures performed were approved by the Walter Reed Army Institute of Research Laboratory Animal Care and Use Committee. Guinea pigs were housed in accordance with requirements listed in the Guide for the Care and Use of Laboratory Animals in an AAALAC, International accredited facility. Guinea pigs were allowed ad libitum access to a 58

standard commercially available diet (Prolab Guinea Pig Animal Diet, PMI Feeds, Inc., St. Louis, MO) and had unlimited access to water. Guinea pigs were housed separately in suspended polycarbonate cages containing recycled paper bedding (Carefresh Bedding, Absorption Corporation, Bellingham, WA). Lights were on a 12-h cycle. Experimental Design and General Procedures: Each guinea pig was randomly assigned to receive one of the following anesthetic regimens: ketamine (Ketaject, Phoenix Pharmaceutical, Inc., St. Joseph, MO), 40 or 60 mg/kg, and xylazine (Rompun, Miles Inc., Agriculture Division, Animal Health Products, Shawnee Mission, KS), 5 mg/kg (KX); ketamine, 40 or 60 mg/ kg, and detomidine (Dormosedan, Pfizer Animal Health, Pfizer Inc., West Chester, PA), 5 mg/kg (KD); ketamine, 40 or 60 mg/ kg, and medetomidine (Domitor, Pfizer Animal Health, Pfizer Inc., Exton, PA), 0.5 mg/kg (KM); tiletamine-zolazepam (Telazol, Fort Dodge Laboratories, Inc., Fort Dodge, IA), 40 mg/kg, and xylazine, 5 mg/kg (TX); tiletamine-zolazepam, 40 mg/kg, and detomidine, 5 mg/kg, (TD); and tiletamine-zolazepam, 40 mg/kg, and medetomidine, 0.5 mg/kg (TM). All anesthetics were administered intramuscularly in the semimembranosus and semitendinosus muscles. Dosages were determined on the basis of reported dosages used by others, including dosages extrapolated from studies that involved other species of rodents. To confine the comparison between anesthetics to results obtained within similar time periods, all anesthetics were tested on each experimental day. A complete set of tests was carried out each day under uniform conditions, thereby distinguishing differences among days from differences attributable to treatment groups and experimental error. To avoid biasing results, investigators were unaware during each day of evaluation of the anesthetic regimen given to a particular guinea pig. A pilot study was initially performed to determine whether remarkable differences existed among the anesthetic regimens and to examine the feasibility of finding regimens with satisfactory success rates. Six guinea pigs were assigned to each group, and results were evaluated after testing three guinea pigs in each group. Failure of all three ketamine regimens to induce a surgical plane of anesthesia caused us to increase the dosage of ketamine to 60 mg/kg. An additional six guinea pigs were then evaluated for each of the ketamine regimens. Six guinea pigs were given each of the six regimens. Surgical success rates exceeding 50% were detected with the TX and TM regimens; hence, additional guinea pigs were allocated to these two groups to enable us to better assess whether desired surgical success rates were possible. The primary variable measured was anesthetic depth, which was evaluated by attempting to perform an ovariohysterectomy. Other variables measured, which were considered secondary endpoints, were time from injection until onset of anesthesia, which was defined as the interval from injection of the anesthetic until loss of righting reflex (ability to regain a sternal position when placed in dorsal or lateral recumbency); duration of anesthesia, defined as the interval from loss of righting reflex until subsequent recovery of the reflex; depth of anesthesia, evaluated by monitoring the ear pinch response and any gross purposeful movements during the surgical procedure; and vital signs (i. e., respiratory rate, heart rate, and body temperature). Vital signs were monitored at 5-min intervals until the guinea pig regained the righting reflex. Before administration of the anesthetic, a baseline heart rate was obtained by means of auscultation, using light manual restraint, and a baseline respiratory rate was obtained by visual observation. After administration of the anesthetic, body temperature was obtained with the use of a probe (IVAC Temp Plus II, model 2017, IVAC Corp., San Diego, CA) inserted in the rectum, heart rate was obtained by use of an ECG monitor (Silogic, model EC60, Jorgensen, Loveland, CO or Datascope, model EL, Datascope Corp., Patient Monitoring Division, Paramus, NJ), and respiratory rate was obtained by visual observation. Surgical Procedure: Once a guinea pig lost the righting reflex, it was placed on a water blanket heating pad (Gaymar T/ Pump, model TP-500, Gaymar Industries Inc., Orchard Park, NY) and attached to an ECG monitor. After five minutes, the depth of anesthesia was initially assessed by evaluation of the ear pinch response. This was performed by using mosquito hemostats that were placed on the pinna and slowly closed to the first ratchet for approximately 1 to 2 sec. If there was response to the stimulus, we waited two minutes and tested the response again. This was repeated for at least eight cycles or until the guinea pig did not have a response, whichever came first. If this noxious stimulus did not elicit a response, hair on the ventral abdomen was clipped, the area was surgically prepared, and a standard abdominal approach was performed. If incising the skin did not elicit a response, a midline incision through the abdominal wall and peritoneum was made, and an ovariohysterectomy performed. The abdominal wall was closed with absorbable suture (3-0 Vicryl, Ethicon, Inc., Somerville, NJ), and staples (AutoSuture Multifire Premium 35W Disposable Skin Stapler, US Surgical Corp., Norwalk, CT) were used to close the skin. At any time during the surgery, if the guinea pig responded (defined as any gross, purposeful movements such as head shaking, leg movement, abdominal tenting, or other signs of pain), we waited five minutes and then attempted to continue the surgery. If the guinea pig still had a response, we waited an additional five minutes. If there was still a response at that time, the incision was closed, and the guinea pig was allowed to recover to the point of reappearance of the righting reflex. All guinea pigs that had any part of the surgical procedure performed were euthanatized after regaining the righting reflex, using CO 2 gas. Guinea pigs that unexpectedly died during or after surgery were submitted for necropsy and histologic evaluation to rule out underlying organic disease. Statistical Analysis: On the basis of analysis of results of the pilot study, a Fisher s exact test was used to determine the number of guinea pigs to be used in those groups for which there was some surgical success. Differences between proportions of guinea pigs on which we successfully completed surgery were evaluated, using a 2 test for homogeneity (STATXACT, Cytel Corp., Cambridge, MA). An analysis of variance with multiple comparisons was performed, using Tukey s pairwise comparison procedure. This method was used in comparing means for interval from injection until onset of anesthesia, duration of anesthesia, and vital signs. Wilcoxon s rank-sum test was used to compare means of vital signs for regimens that had surgical success. Tables of summary descriptive statistics were constructed to contrast basic data of interest. Differences were considered significant at P < 0.05. Results We successfully completed surgery on 14 of 30 (46.7%) guinea pigs in the TX group (95% confidence limits [CL] = 28.3, 65.7) and 23 of 29 (79.3%) guinea pigs in the TM group (95% CL = 60.3, 92.0). Based on the 2 test for homogeneity, the proportion of guinea pigs on which we completed surgery was significantly (P = 0.01) higher for the TM group. Odds that a guinea pig given TM would undergo successful surgery was 4.4 times that of an animal given TX (95% CL = 1.4, 13.8). All regimens that included ketamine as the dissociative anesthetic as well as both detomidine regimens failed to provide adequate analgesia to enable us to perform an ovariohysterectomy. Time from injection until onset of anesthesia ranged from 20 59

Table 1. Duration and time to onset of anesthesia (minutes) Anesthetic regimen n Time to onset of anesthesia Duration of anesthesia a Mean Range Mean Range KX KD 40 mg/kg 3 1.4 1.0 2.3 104.3 76 137 60 mg/kg 6 1.6 0.3 3.0 91.0 80 103 40 mg/kg 3 1.8 1.0 3.0 132.0 110 157 60 mg/kg 6 1.1 0.7 2.0 124.5 29 167 KM 40 mg/kg 3 1.7 1.5 2.0 156.7 133 196 60 mg/kg 6 1.0 1.0 1.0 145.5 118 164 TX 30 1.2 0.5 3.0 131.3 94 222 surgery completed 14 1.3 0.5 3.0 130.8 94 155 not completed 16 1.2 1.0 2.0 131.7 116 222 TD 6 0.9 0.5 1.0 217.3 b 191 234 b TM 29 1.3 0.5 3.0 196.3 152 251 surgery completed 23 1.3 1.0 2.0 195.3 152 251 not completed 6 1.3 0.5 3.0 200.5 168 230 a Values for TM and TD were significantly (P < 0.001) greater than for TX, KM, KD, or KX. b Represents only three guinea pigs that recovered from anesthesia. KX = ketamine and xylazine, KD = ketamine and detomidine, KM = ketamine and medetomidine, TX = tiletamine-zolazepam and xylazine, TD = tiletamine-zolazepam and detomidine, TM = tiletamine-zolazepam and medetomidine. Table 2. Mean heart rate determined at three time points Anesthetic regimen n Time point a Onset of anesthesia b Minimum value c Recovery from anesthesia KX KD 40 mg/kg 3 214 21 173 9 181 4 60 mg/kg 6 206 16 148 13 155 22 40 mg/kg 3 226 13 187 14 199 22 60 mg/kg 6 217 9 155 8 161 8 KM 40 mg/kg 3 205 17 192 12 196 16 60 mg/kg 6 197 19 155 17 160 18 TX 30 209 25 152 22 169 27 TD 6 239 12 172 28 d 195 42 e TM 29 214 21 142 24 172 34 a Data expressed as mean S.D. b Values for TD are significantly (P = 0.002) greater than for TX and KM60. c Values for TM are significantly (P = 0.002) less than for KD40 and KM40. d Represents mean of four guinea pigs. e Represents mean of only three guinea pigs that recovered from anesthesia. See Table 1 for key. sec to 3 min for all guinea pigs, which did not differ significantly among regimens (Table 1). The TM and TD regimens had the greatest mean times for duration of anesthesia and were similar to each other, but were significantly different (P < 0.001) from all other regimens. Comparing the only two regimens for which we were able to successfully complete surgery, the duration of anesthesia for TM (196.3 min) was significantly (P < 0.001) longer than that for TX (131.3 min). None of the guinea pigs administered detomidine lost the ear pinch response. Two of nine (22%) guinea pigs in the KX group and three of nine (33%) in the KM group lost the ear pinch response, and surgery was attempted, but it could not be successfully completed. Additionally 3 of 30 (10%) guinea pigs in the TX group and 3 of 29 (10.3%) guinea pigs in the TM group lost the ear pinch response, but we were unable to successfully complete surgery. This resulted in 11 of 77 (14.3%) guinea pigs among these four groups in which surgery was attempted. Some reacted to incising the skin, whereas others did not react until the viscera was manipulated. For evaluating heart rate, respiratory rate, and body temperature, three time points were considered: five minutes after onset of anesthesia (loss of righting reflex), the point at which minimum value was attained during anesthesia, and recovery from anesthesia (recovery of righting reflex). For the two groups (TX and TM) for which we had surgical success, mean heart rate did not significantly differ among the three time points (Table 2). At all three time points, guinea pigs in the TD and KD groups had higher mean respiratory rates than the other groups (Table 3). Guinea pigs that successfully underwent a complete surgery, regardless of the anesthetic regimen administered, had a significantly (P < 0.001) lower mean minimum heart rate (142 23.6 beats/min), compared with those that did not undergo a complete surgery (160.9 22.5 beats/min). However, mean respiratory rate observed at the time of the minimum heart rate did not significantly differ between these two groups (successfully completed = 54.6 14.3 breaths/min; not completed = 64.1 41.3 breaths/min). Mean body temperature did not differ significantly among 60

Table 3. Mean respiratory rate at three time points Anesthetic regimen n Time point a Onset of anesthesia b Minimum value c Recovery from anesthesia d KX KD 40 mg/kg 3 88 4 43 13 55 13 60 mg/kg 6 80 17 43 8 48 10 40 mg/kg 3 125 53 80 36 101 44 60 mg/kg 6 123 46 64 41 121 68 KM 40 mg/kg 3 61 2 43 2 48 4 60 mg/kg 6 59 7 38 7 48 14 TX 30 75 22 47 14 64 24 TD 6 169 44 69 14 e 112 59 f TM 29 76 18 44 13 63 16 a Data expressed as mean S. D. b Values for TD are significantly (P = 0.002) greater than for TM, TX, KX or KM. c Values for KD are significantly (P = 0.002) greater than for TM, TX, and KM; Values for KD40 are significantly (P = 0.002) greater than for TX, TM, and KM60. d Values for TD are significantly (P = 0.002) greater than for KX60 and KM60; values for KD60 are significantly (P = 0.002) greater than for TM, TX, KX and KM. e Represents mean of four guinea pigs. f Represents only three guinea pigs that recovered from anesthesia. See Table 1 for key. anesthetic groups at any of the three time points. There was not a significant difference in mean minimum body temperatures in guinea pigs on which we successfully completed surgery (95.1 F [35.1 C]) versus those on which we did not (94.5 F [34.7 C]), regardless of anesthetic regimen. Mean body temperatures for the six groups at the three time points ranged from 94.2 to 101.9 F (34.6 to 38.8 C). Minimum body temperature among guinea pigs ranged from 86.4 to 100.7 F (30.2 to 38.2 C). Low heart rate (LHR) and low body temperature (LBT) were defined as values that were one S. D. greater than the mean of the minimum for that particular variable. On the basis of this definition, LHR was determined to be 170 beats/minute and LBT was 97.7 F (36.5 C). Comparing the two groups for which we had surgical success, mean time to reach LHR for the TM group (26 min) was similar to that for the TX group (30 min). However, mean duration of LHR for the TM group (118 min) was significantly (P = 0.007) greater than that for the TX group (69 min). Mean time to reach LBT for the TM group (25 min) was significantly (P = 0.02) less than that for the TX group (31 min), whereas mean duration of LBT for the TM group (128 min) was significantly (P = 0.001) greater than that for the TX group (69 min). Few adverse effects were detected for most of the anesthetic regimens. The exceptions were the detomidine groups. Four of nine guinea pigs in the KD groups (initial pilot study and increased dosages) were tachypneic (mean respiratory rate greater than 100 breaths/minute). Fourteen guinea pigs had varying amounts of gastric reflux with gastric contents being expelled via the nostrils and mouth (KD, 9/9; TD, 3/6; TM, 2/29). Five of the six guinea pigs in the TD group had respiratory difficulty as evidenced by tachypnea and varying degrees of cyanosis, and four of these five died or were euthanatized. Two guinea pigs in this group had epistaxis just before they died (one died 13 min after and the other died 30 min after anesthetic administration). One guinea pig was found dead in its cage and was estimated to have died 22 h after anesthetic administration. Although this guinea pig had many of the signs that the others in the TD group had, it recovered from anesthesia and did not exhibit any apparent distress at the time of recovery. The fourth guinea pig appeared to be in respiratory distress and was not allowed to recover from anesthesia; it was euthanatized three hours after anesthetic administration. Necropsy and microscopic examination revealed respiratory failure due to bronchopneumonia as the cause of death for the guinea pig that died 22 h after anesthetic administration. There was plant material in the airways which indicated aspiration of gastric contents as the cause of pneumonia. A complete necropsy and histologic examination of the other three guinea pigs with respiratory distress revealed pulmonary edema or hemorrhage in all three. It was reported that the hemorrhage was most likely a result of terminal dyspnea. Discussion The only anesthetic regimens that provided anesthesia and analgesia sufficient to enable us to perform a major surgical procedure were TX and TM. All ketamine combinations were inadequate. Even increasing the dosage by half did not result in surgical success. Our results support other findings that KX and KM, at the dosages used in this study, are inadequate to allow a major surgical procedure (9,15). Barzago et al. reported that a combination of ketamine (87 mg/kg) and xylazine (13 mg/kg) was effective for use in performing minor surgical procedures (20). However, most of those guinea pigs required additional doses of ketamine to maintain adequate analgesia. In another study, a combination of ketamine and medetomidine was suitable in guinea pigs only for immobilization (15). Detomidine appears to be unacceptable for use in guinea pigs at the dosage used in the study reported here. Rats given the same dosage of KD as that used in this study were successfully anesthetized and had respiratory rates that were similar to control (saline-treated) rats (21). However, five of six rabbits administered varying regimens of ketamine and detomidine developed myocardial necrosis (18). In our guinea pigs, there was no evidence of myocardial necrosis, but there was a high rate of adverse effects including gastric reflux, tachypnea, cyanosis, and death. In addition, none of the guinea pigs given detomidine lost its ear pinch response, and, thus, none proceeded to surgery. Although the mean time from injection until onset of anesthesia was similar among all anesthetic regimens, mean duration of anesthesia varied from 90 min to more than 3 h. Duration of anesthesia did not necessarily correlate with depth of anesthesia. Within the TM and TX groups, mean duration of anesthesia was almost identical between those on which we successfully completed surgery and those on which we did not. Although duration of anesthesia was significantly longer for the TM group (more than three hours), compared with the TX group, this was not necessarily advantageous. Whereas it may be useful for prolonged 61

procedures, many surgical procedures require less than 1 h, and therefore extra time and manpower would be required to monitor these animals until they fully recover. However, atipamezole hydrochloride (Antisedan, Pfizer Animal Health, Pfizer Inc., Exton, PA), an 2 -adrenoreceptor antagonist that reverses the effects of medetomidine, could be given to decrease the duration of anesthesia. Duration of the actual surgical procedure ranged from 10 to 20 min. Although duration of anesthesia was at least 90 min for all guinea pigs that survived, we could not determine from this study the actual duration of surgical anesthesia. Both TD and KD groups had mean respiratory rates that were greater than for all the other groups at each of three indicated time points. This reflected the increased incidence of tachypnea that was observed in the guinea pigs given either of these anesthetic regimens. Mean body temperature did not significantly differ between the groups. Mean minimum heart rate achieved during the anesthetic episode correlated with the ability to successfully survive until completion of surgery. Guinea pigs that successfully underwent complete surgery, regardless of the anesthetic regimen administered, had a lower mean minimum heart rate, compared with those that did not successfully undergo surgery. However, this was well after completion of the surgical procedure and, thus, cannot be used as a predictor of ability to successfully survive surgery. Medetomidine appears to have a more powerful effect on decreasing heart rate and body temperature than does xylazine. Although mean minimum body temperature did not differ significantly among groups, TM had a more powerful effect in keeping the body temperature low for longer periods, compared to TX. This effect was evident regardless of surgical outcome. It is also interesting that, among the two groups for which we had any surgical success, once the body temperature decreased to less than 97.7 F (36.5 C), at least half of the guinea pigs in each group never reattained that body temperature before recovering from anesthesia. Even though all guinea pigs were maintained on a water heating blanket, some became severely hypothermic. This underscores the importance of maintaining supplemental heat for these animals during anesthesia as well as during the postanesthetic period. In addition to a water heating blanket, heat lamps or warm water bottles may be necessary to provide supplemental heat to prevent hypothermia during anesthesia. Although investigators in other studies have used multiple reflexes to assess anesthesia and analgesia (3,9,11,13,14,16), we used the ear pinch response as an indicator of analgesia and depth of anesthesia. The ear pinch response is considered to be the most sensitive reflex in guinea pigs (15). Smith evaluated the righting, cutaneous, pedal, corneal, pupillary, palpebral, and laryngeal reflexes in guinea pigs anesthetized with varying dosages of ketamine and xylazine (16). The dosage reported as necessary to abolish responses to external stimuli (a combination consisting of 52.5 mg of ketamine/kg and 7.5 mg of xylazine/ kg) was similar to our increased dosage (60 mg of ketamine/kg and 5 mg of xylazine/kg). However, we found that four of six guinea pigs never lost the ear pinch response. Green et al. reported that pain and corneal reflexes were retained in guinea pigs when they used a combination of 60 mg of ketamine/kg and 8 mg of xylazine/kg (9). Other reports reveal that once certain reflexes such as ear pinch response, pedal, and righting reflexes disappear, surgical anesthesia has been established (14); however, results of our study disagreed with this assessment. Eleven guinea pigs lost the ear pinch response, yet reacted to surgical manipulation; the anesthetic regimen did not provide adequate analgesia to complete the ovariohysterectomy. Discrepancies exist in the literature regarding anesthesia of guinea pigs (4 6,9), but much of this could be due to major differences between studies. Differences in age, weight, sex, or strain of guinea pig, route of injection, or use of supplemental heat can lead to differences in anesthetic outcome. However, information regarding these factors is frequently unavailable. These are potentially important variables to consider when determining response of a guinea pig to an anesthetic regimen. Analysis of results of the study reported here indicated that detomidine, at the dosage used, was inappropriate for use in guinea pigs. 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